Plants growing at the bottom of giant sinkholes in China are so rich in nutrients that they grow faster than their surface-dwelling counterparts while using fewer basic building blocks, a new study finds.
The sinkholes called “tiankeng” are one of the last remaining natural refuges for ancient forests and may harbor species unknown to science – but exactly how these species could thrive at the bottom of these deep pits was not clear.
It turns out that the laurels, nettles, and ferns that live in tiankeng thrive on vast supplies of nitrogen, phosphorus, potassium, calcium, and magnesium, all of which limit plant growth in other environments where they are scarce. But since these nutrients are abundant in tiankeng, plants drink them so they can grow tall and make the most of the bits of sunlight that reach them, according to research published online July 20 in Chinese Journal of Plant Ecology.
“Plants can adapt to adverse environments by adjusting nutrient content,” the researchers wrote in the study, which was translated from Mandarin using Google Translate.
Very little light reaches the bottom of the tiankeng, which means “sky pits” in Mandarin. The Tiankeng are 330 feet (100 meters) deep sinkholes in the southwestern karst landscape of China. These deep pits harbor plants that favor moisture and shade, including species unique to the region, according to the study.
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“Due to the towering cliffs and steep terrain of the tiankeng, it has been less disturbed by human activities,” the researchers wrote. The Tiankeng are refuges for modern karst forest plants, including the Nepalese pig plum (Choerospondias axillaris) and the Chinese rain bell (Strobilanthes cusia), they wrote.
For the study, researchers collected samples from 64 plant species in and outside tiankeng in Leye County, in China’s Guangxi region. Leiye County is home to the Dashiwei Tiankeng Group, a geological wonder including 30 sinkholes in a landscape covering 7.7 square miles (20 square kilometers). To determine whether the nutrient uptake and growth strategies of these plants differed depending on their environment, the team measured the carbon and nutrient content of each sample.
Plants growing inside the tiankeng had lower carbon content than those growing outside, but had higher levels of every other element the researchers measured, such as calcium and potassium, as well as higher growth rates.
Carbon is essential to plants, making up much of their “skeletons” and structures that improve water retention, according to the study. But the humid conditions inside the tiankeng mean the plants do well with lower levels of carbon in their tissues because they don’t need to retain as much water. Plants growing on the surface contain more carbon, probably because “the forest outside the pit has high light intensity, rapid water evaporation, poor soil, greater interference from human activities, and easy soil loss,” the researchers wrote.
Compared to surface plants, plants growing inside the tiankeng have higher levels of nitrogen and phosphorus, both of which the plants draw from the soil. The soil at the bottom of the tiankeng contains more of these elements than the surface soils, indicating that plants absorb them more easily. Karst soil is rich in calcium and magnesium, and tiankeng plants show much higher levels of these elements than surface plants. They also have higher levels of potassium, although potassium is relatively scarce in karst soils.
Tiankeng plants absorb nutrients more easily than “conservative” surface plants because nutrients are more abundant in the shady depths of the holes and because the plants have to grow taller, according to the study.
“The nutrient status of the soil in the tiankeng forest is good,” the researchers wrote, and the plants have evolved to make the most of the available resources to grow quickly and gather more light.
